Let us set aside the question about whether God takes sides in American elections. The more pressing fact about this kind of religious-political advertising is that you, dear taxpayer, are footing part of the bill.
According to a recent Pew survey, about 13% of regular churchgoers reported receiving "political information" at church services, and about half of those said they were urged to support a specific candidate or party. So, to use the language of marketing, church-based advertising reached somewhere between 8 and 16 million "eyeballs".
If candidates had to buy this kind of advertising – and they surely would if they could – it would cost them tens, or perhaps hundreds, of millions of dollars over the course of a campaign. And, of course, the money they raised and spent would all be subject to tax.
Religious organizations, on the other hand, are largely exempt from taxes.
Now, tax exemptions are really just another name for a public subsidy. If I don't pay my share of public expenditure on roads, law enforcement, defense and so on, the difference has to come from you. And it is for precisely this reason that the law forbids religious organizations, like all similar tax-exempt groups, from direct involvement in electioneering.